Electric potential difference and Ohm's law review

Ohm’s law states that for some devices there is a relationship between electric potential difference, current, and resistance.

The equation is: $I={\displaystyle \frac{\mathrm{\Delta }V}{R}}$‍

Where $I$‍ is current, $\mathrm{\Delta }V$‍ is electric potential difference, and $R$‍ is resistance.

For a given resistance $R$‍, increasing the electric potential difference $\mathrm{\Delta }V$‍ increases the current $I$‍ and vice versa.

For a given electric potential difference $\mathrm{\Delta }V$‍, if the resistance $R$‍ increases, then the current $I$‍ decreases and vice versa.

For a given current $I$‍, if the electric potential difference $\mathrm{\Delta }V$‍ increases, then the resistance $R$‍ also increases and vice versa.

If the current encounters resistance, the electric potential difference decreases according to Ohm’s law. We sometimes call this a voltage drop.

A common source of electric potential is a battery, which is represented in diagrams by the symbol below (Figure 2). The short side is the negative end, with a lower electric potential, and the long side is the positive end, with a higher electric potential.

Electrons flow from the negative terminal to the positive terminal. Conventional current $I$‍ travels from the positive terminal (higher electric potential), through the circuit, and finally to the negative end (lower electric potential).

Current flow and electric potential difference can be better understood by using the analogy of a boulder rolling down a hill. At the top of the hill, the boulder has a lot of gravitational potential energy. Similarly, an electron has a lot of stored energy in the form of electric potential energy when it is at the negative terminal of a battery. The boulder will naturally fall toward the ground where potential energy is lower. The electron at the negative terminal of a battery will naturally flow toward the positive terminal, where the electric potential is lower.

As the boulder falls downward, the stored energy is converted to kinetic energy. As the electron flows across electrical components, the stored energy is converted into various forms of energy such as heat and light.

Sometimes people think all devices follow Ohm’s law. However, a device is only ohmic when the current is directly proportional to the electric potential difference, and inversely proportional to the resistance. If we plotted an electric potential vs. current graph for an ohmic device, the relationship would be linear (see Figure 3).

Some devices such as light bulbs are non-ohmic. This means that their electric potential difference-current graphs are non-linear, as in Figure 3. For non-ohmic devices, we can’t use $I={\displaystyle \frac{\mathrm{\Delta }V}{R}}$‍ to solve for an unknown.

For deeper explanations on electric potential and Ohm's law, see our video on circuits and Ohm's law.

To check your understanding and work toward mastering these concepts, check out the exercise on Ohm's law.